Session 13.3 Embedded cellular convection in three-dimensional moist flow past two-dimensional topography

Thursday, 24 June 2004: 11:15 AM
Oliver Fuhrer, ETH, Zurich, Switzerland; and C. Schär

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The occurrence, structure and impact of embedded cellular convection in orographic precipitation is investigated. To this end, idealized three-dimensional simulations of moist flow past a two-dimensional ridge are performed with a convection resolving cloud model. It is found that the convective dynamics can substantially alter the precipitation efficiency and distribution of orographic precipitation.

Statically unstable cloud regions are shown to be a necessary but not sufficient ingredient for the growth of convective elements. These regions are in general not known a priori, since they are a function of the nonlinear flow dynamics. Using idealized atmospheric profiles of constant dry stratification (Nd=0.01/s) and relative humidity (RH=95%), key factors controlling the development of cellularity are investigated: potential instability of the upstream profile, wind speed and vertical wind shear.

It is shown that - in addition to the upstream ambient conditions - the underlying topography also plays an important role. Recent experimental evidence indicates that small-scale topographic features may dominate the local distribution of cloud and precipitation in complex terrain. To test this idea, the sensitivity of the above results to irregularities in the topography is investigated, using the the same idealized simulation setup but superimposing small-scale topographic features onto the ridge. Preliminary results indicate that especially in the case of shallow cellular convection the precipitation distribution is influenced by small-scale topography.

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